1. Field of the Invention
The present invention relates generally to a magnetic tape cassette. More particularly, it relates to a tapping boss of the upper half casing of a magnetic tape cassette which is formed by fastening the upper half casing and a lower half casing by tapping screws.
2. Description of the Prior Arts
A magnetic tape cassette or a cartridge for a video tape generally comprises a pair of upper and lower half casings in which a magnetic recording tape wound around hubs is held. It is, therefore, necessary to fasten the upper and lower half casings by tapping screws after both the casings are coupled. For this, tapping bosses for receiving the screws and the corresponding number of boss supporters brought into contact with the tapping bosses are set up from the base plate of respective upper and lower half casings at each corner and other suitable positions.
FIG. 1 shows a tapping boss (2) of the upper half casing of the conventional magnetic tape cassette for video tape by a solid line and a boss supporter (4) of the lower half casing which is brought into contact with the end of the tapping boss (2) by a imaginary line.
The boss supporter (4) of the lower half casing comprises a holder (8) of a cylindrical body having a gradually reduced end which is formed on the base plate (6) of the lower half casing. A recess (12) is formed at the top end (10) of the holder (8) and a through hole (18) is formed between the recess (12) and a counter sinking cavity (15) formed in the outer surface of the base plate (6) to receive the head (14) of the tapping screw so that the body portion (16) of the tapping screw is inserted through the hole (18).
The tapping boss of the upper half casing comprises a stem (22) of a cylindrical body having a gradually reduced end which is formed on the base plate (20) of the upper half casing in one piece and a resilient projection (24) which projects from the top of the stem (22) and is capable of snapping engagement with the recess (12) of the holder (8) of the lower half casing thereby determining positions of the upper and lower half casings. A threaded hole (26) is formed in the stem so as to engage with the body portion (16) of the tapping screw. The resilient projection (24) comprises four thin pieces (24a) to (24d) which are formed by dividing a thin-walled tubular body surrounding the threaded hole (26) into four parts and each thin piece is resiliently deformable inwards. The resilient projection (24) has an outer diameter slightly greater than the inner diameter of the recess (12) of the holder (8) so that when the upper and lower half casings are assembled, the top end of the projection comes into contact with a chamfered edge of the recess (12) and then is snap-fitted into the recess by resilient deformation. An annular surface region on the top of the stem from which the resilient projection projects provides a shoulder (30) brought into contact with the top (10) of the boss supporter of the lower half casing.
In the conventional magnetic tape cassette, the chamfered portion (28) formed around the edge of the recess (12) of the boss supporter (8) ensures easy insertion of the resilient projection; however, this requires for the top of the tapping boss (2) an annular region, i.e. a contact shoulder (30) which should be out of the chamfered portion (28). It is, therefore, necessary for the stem (22) to form a cylindrical body having a gradually reduced end which should have the thickness required to receive the tapping screw . This configuration often causes a large shrinking recess (32) on the outer surface of the base plate (20) at the point to which the root of the stem (22), not having a reduced thickness such as that of the counter sinking hole (16) of the lower half casing is connected. The large shrinking recess (32) causes problems in that accuracy of shape of the magnetic tape cassette such as parallelism and flatness is greatly reduced and appearance is inferior.
One attempt to overcome the problem of the shrinking recess is to reduce the diameter of the stem of the tapping boss (2). However, there is an disadvantage in that the surface area of the contact shoulder (30) is reduced resulting in insufficient support of the holder (4) to the tapping boss (2).